How can I get the full path of the selected file from the file explorer?
bpy.path.abspath()

Returns the absolute path relative to the current blend file using the “//” prefix.

TLDR; Just call bpy.path.abspath(your_path) to get the absolute path representation.

There is no reason to display the absolute path nor convert it because the user can set it to absolute or relative in the File Browser (N):

Also notice: there are several naming conventions like adding a separator to the name of your panel class or using lowercase names for functions and variables (PEP8):
import bpy

class CATEGORY_PT_CustomPanel(bpy.types.Panel):
    bl_idname = &quot;OBJECT_PT_my_panel&quot;
    bl_label = &quot;My Panel&quot;
    bl_space_type = &quot;VIEW_3D&quot;   
    bl_region_type = &quot;UI&quot;
    bl_category = &quot;Tools&quot;
    bl_context = &quot;objectmode&quot;

    def draw(self, context):
        layout = self.layout
        scn = context.scene

        col = layout.column(align=True)
        col.prop(scn, &quot;my_path&quot;, text=&quot;&quot;)

        # Print the absolute and relative path to the console
        print (&quot;REL:&quot;, scn.my_path)
        print (&quot;ABS:&quot;, bpy.path.abspath(scn.my_path))


def register():
    bpy.utils.register_class(CATEGORY_PT_CustomPanel)
    bpy.types.Scene.my_path = bpy.props.StringProperty(name=&quot;File&quot;, subtype='FILE_PATH')

def unregister():
    bpy.utils.unregister_class(CATEGORY_PT_CustomPanel)
    del bpy.types.Scene.my_path


if __name__ == &quot;__main__&quot;:
    register()

Related

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